Separation of air



Dec. 8, 1959 P. M. SCHUFTAN ET AL 2,915,382

SEPARATION OF AIR Filed April 23, 1956 RESSOR I nventor Attorney 2,915,882 SEPARATION or AIR Paul Maurice Schuftan, Richmond Hill, and Anthony Duncan Littlwood, Wbiidfoid Green, England, as-

ors to The British Oxygen Company Limited, a British company Application April 23, 1956, Serial No. 579,932 Claims priority, application Great Britain May 31, 1955 Claims. (Cl. 62-30) The present invention relates to air separation and more particularly to a process of the type in which air is compressed, cooled, expanded and subjected to rectification to produce a liquid oxygen fraction and a gaseous nitrogen fraction.

The oxygen fraction is frequently required to be delivered in the gaseous phase under pressure and it has been proposed to pump the liquid oxygen fraction under pressure and to vaporize the liquid by heat interchange with the incoming air, the whole of the air to be separated being compressed to a uniform and relatively high pressure.

It is an object of this invention to provide for producing a compressed gaseous oxygen fraction, an air separation process having enhanced thermodynamic efliciency as compared with other processes of this type hitherto proposed.

According to this invention, an air separation process of the type specified comprises establishing a major air stream and a minor air stream, compressing the major air stream only to a relatively low pressure sufficient on expansion with the performance of external work to cover the greater part of the cold requirements of the process, compressing the minor stream to a relatively high pressure, sufficient to recover by heat exchange the cold conent of the separated compressed liquid oxygen fraction, and passing the compressed minor air stream in heat exchange relationship With the compressed liquid oxygen fraction prior to the expansion of the minor air stream.

Preferably the whole of the major stream is expanded with the performance of external work, for example, in a turbine, while the minor stream, after being cooled by heat transfer with the compressed liquid oxygen fraction, is expanded isenthalpically. The two streams after expansion are passed into a double rectification system operating in known manner to produce the required liquid oxygen fraction and a gaseous nitrogen fraction.

The invention will now be more particularly described with reference to the accompanying drawing which shows diagrammatically one method of carrying out the invention.

Air is drawn into the compressor 1 through a pipe 2, compressed therein to about atmospheres absolute pressure, and then passed through a vessel 3 where carbon dioxide is removed by scrubbing, into exchanger 4 where the air is cooled to a temperature of about 253 K. with simultaneous removal of water vapour, the cooling and drying agent being the separated gaseous nitrogen fraction. In the drawing, the whole of the gaseous nitrogen fraction is shown as passing through the exchanger 4, but, if desired, only a part of this fraction may be so treated.

The air leaving the exchanger 4 is then split into two streams. One stream is further compressed to about 80 atmospheres absolute pressure in a compressor 5 and dried in vessel 6 from which it passes through an exchanger 7 in countercurrent flow to the separated liquid V United States Patent 0 2,915,882 Patented Dec. 8, 1959' ice oxygen fraction which cools it to a temperature of about 116 K. This air is further cooled in an exchanger 8' separated gaseous nitrogen fraction, leaving them at a. temperature of about 213 K. and is further cooled to about 118 K. in an exchanger 13 by the separated gas eous nitrogen fraction. This air stream is finally expanded in the turbine 14 to a pressure of 5.5 atmos= pheres absolute and a temperature of about 100 K. and.

is introduced into the scrubber 10 to admix With the first.

air stream.

From the scrubber 10 the liquid and vapour fractions of the cooled and expanded air are introduced into the upper and lower columns 16 and 15 respectively, of at double column system, to efiect separation into a liquid. oxygen fraction and a gaseous nitrogen fraction in known:

manner.

The gaseous nitrogen fraction leaving the top of the upper column 16 is first used to cool the liquid nitrogen: stream leaving the lower column 15 to act as reflux in: the upper column 16, in an exchanger 17 in the manner well known in the art. The effluent gaseous nitrogen fraction then passes through the exchangers 8, 13, 11 or 12 and 4 as hereinbefore described in order to cool the incoming air streams.

If a pure nitrogen fraction is required, this can be obtained from the top of the upper column 16 by withdrawing an impure nitrogen stream from a lower position in the upper column in known manner. The two nitrogen streams are then directed through separate sets of exchangers in a similar manner to that hereinbefore described.

The liquid oxygen fraction withdrawn from the bottom of the upper column 16 is compressed to about 20 atmospheres absolute pressure by means of a liquid pump 18, and the compressed liquid is vaporized and the resulting vapour heated in the exchanger 7 by heat exchange with the stream of air compressed to atmospheres absolute pressure as previously described.

By this means, a gaseous oxygen stream at a desired pressure is obtained in a highly efficient manner.

We claim:

1. Process for the separation of air to produce a compressed gaseous oxygen fraction and a gaseous nitrogen fraction which comprises the steps of compressing the air to a relatively low pressure, cooling the compressed air by heat exchange with at least a part of at least one of the separation products, dividing said cooled air into a major air stream and a minor air stream, expanding the whole of said major air stream with the production of external work, whereby an amount of cold is produced suflicient to satisfy the greater part of the cold requirements ofthe process, compressing said minor air stream to a relatively high pressure, cooling the whole of said compressed minor air stream by heat exchange with a liquid oxygen fraction pumped under pressure, whereby said liquid oxygen fraction is vaporized to produce the required compressed gaseous oxygen fraction, expanding said cooled minor air stream, subjecting said expanded major and minor air streams to rectification to separate therefrom a gaseous nitrogen fraction and a liquid oxygen fraction and pumping said liquid oxygen fraction under pressure to produce the liquid oxygen fraction which is vaporized by heat exchange with the compressed minor air stream.

2. Process for the separation of air to produce a compressed gaseous oxygen fraction and a gaseous nitrogen sufiicient to. satisfy the greater part of the cold requirements of the process, compressing said minor air stream :to a relatively high pressure, cooling the whole of said'mie nor air stream byheat exchangewith a liquid oxygen frac- .tion pumped under pressure, whereby said liquid oxy- -gen fraction is vaporized to produce the required compressed gaseous oxygen fraction,,isenthalpically expanding said cooled minor air stream, subjecting said expanded major and minor air streams to rectification to separate therefrom a gaseous nitrogen fraction and a liquid, oxygen fraction and pumping said liquid oxygen fraction under pressure to produce the liquid oxygen fraction which is vaporized by heat exchange with the compressed minor air stream.

Q3. Process for the separation of air to produce a compressed gaseous oxygen fraction and a gaseous nitrogen fraction which comprises the steps of compressing the air to a relatively low pressure, cooling the compressed air by heat exchange with at least a part of the separated gaseous nitrogen fraction, dividing said cooled air into a major air stream and a minor air stream, expand- 1 ing the whole of said major air stream with the produc tion of external work, whereby an amount of coldris pro-- dluced sufficient to satisfy the greater part of. the cold requirements of the process, compressing said minor air :stream to a relatively high pressure, cooling the whole of said minor air stream by heat exchange with a liquid oxygenfraction pumped under pressure, whereby said liquid oxygen fraction is vaporized to produce the required compressed gaseous oxygen fraction, isenthalpical- 1y expanding said cooled minor air stream, subjecting said expanded major and minor air streams to rectification to separate therefrom a gaseous nitrogen fraction and a liquid oxygen fraction and pumping said liquid oxygen fraction under pressure to produce the liquid oxygen fraction which is vaporized byheat exchange with, the compressed minor air stream.

4. Process for the separation of air to produce a com pressed gaseous oxygen fraction and a gaseous nitrogen fraction which comprises the steps of compressing, the air to a relatively low pressure, cooling the compressed .air by heat exchange with at least a part of the separated gaseous nitrogen fraction dividing said cooled'air into a major air stream and a minor air stream, furthercooling said major air stream by heat exchange with said separated gaseous nitrogen fraction, expanding the .whole of said cooled major air stream with the productionof external work, whereby an amount of cold isrproduced sufiicient to, satisfy the greater part of the cold require-,.

ments of the process, compressing said minor air stream to a relatively high pressure, cooling the whole of said compressed minor air stream by heat exchange with a liquid oxygen fraction pumped. under pressure, wherev by said liquid oxygen fraction is vaporized to produce the required compressed gaseous .oxygen fraction, isenthalpically expanding said cooled minor air stream, subjecting said expanded major and minor air streams to rectification to separate therefrom a gaseous nitrogen fraction and a liquid oxygen fraction and pumping said liquid oxygen fraction under pressure to produce the 7 liquid oxygen fraction which isvaporized by heat exchange with the compressed minor air stream. V

5. Process for the separation of air to produce a compressed gaseous oxygen fraction and-a gaseous nitrogen fraction which comprises the steps of compressing the air to a relatively low pressure, cooling the compressed air 'by heat exchange with at least a part of the separatedgaseous nitrogen fraction dividing sa'id cooled air'into a major air stream and; a minor air stream, further cooling said major air stream by heat exchange with said:

separatedgaseous nitrogen fraction, expanding the whole of said cooled major air stream with the production of external work, whereby an amount of cold is'produced'.

sufiicient to satisfy the greater part of the cold requirements of theprocess, compressing said minor air stream to 3.,I'tilfltlV6lY- high pressure, cooling the whole of said minor air stream by heat exchangewith a liquid oxygen fraction pumped under pressure, whereby said liquid oxygen fraction is vaporized to produce the required compressed gaseous oxygen fraction, further cooling said. compressed minor air stream by heat exchange with said! separated gaseous nitrogen fraction, isenthalpically ex panding said cooled-minor air stream, subjecting said expanded major and minor air streams to rectification to separate therefrom a gaseous nitrogen fraction and a: liquid oxygen fraction and pumping said liquid oxygen fraction under pressure to produce the liquid. oxygen fraction which is vaporized by heat exchange with the compressed minor air stream.

References Cited in the file of this patent UNITED STATES PATENTS 2,209,748 Schlitt July 30, 19.40 2,239,883 Bauflre et'al. Apr. 29, 1941 2,409,458 Van Nuys Oct. 15, 1946 2,411,680 Dennis Nov. 26, 1946 2,685,183 Collins Aug. 3, 1954 2,699,047 Karwat Jan. 11, 1955' 2,712,738 Wucherer July 12, 1955 FOREIGN PATENTS 4 6 9,939 Great Britain Aug. 3, 1937 

1. PROCESS FOR THE SEPARATION OF AIR TO PRODUCE A COMPRESSED GASEOUS OXYGEN FRACTION AND A GASEOUS NITROGEN FRACTION WHICH COMPRISES THE STEPS OF COMPRESSING THE AIR TO A RELATIVELY LOW PRESSURE, COOLING THE COMPRESSED AIR BY HEAT EXCHANGE WITH AT LEAST A PART OF AT LEAST ONE OF THE SEPARATION PRODUCTS, DIVIDING SAID COOLED AIR INTO A MAJOR AIR STREAM AND A MINOR AIR STREAM EXPANDING THE WHOLE OF SAID MAJOR AIR STREAM WITH THE PRODUCTION OF EXTERNAL WORK, WHEREBY AN AMOUNT OF COLD IS PRODUCED SUFFICIENT TO SATISFY THE GREATER PART OF THE COLD REQUIREMENTS OF THE PROCESS, COMPRESSING SAID MINOR AIR STREAM TO A RELATIVELY HIGH PRESSURE, COOLING THE WHOLE OF SAID COMPRESSED MINOR AIR STREAM BY HEAT EXCHANGE WITH A LIQUID OXYGEN FRACTION PUMPED UNDER THE PRESSURE, WHEREBY SAID LIQUID OXYGEN FRACTION IS VAPORIZED TO PRODUCE THE REQUIRED COMPRESSED GASEOUS OXYGEN FRACTION, EXPANDING SAID COOLED MINOR AIR STREAM, SUBJECTING SAID EXPANDED MAJOR AND MINOR AIR STREAMS TO RECTIFICATION TO SEPARATE THEREFROM A GASEOUS NITROGEN FRACTION AND A LIQUID OXYGEN FRACTION AND PUMPING SAID LIQUID OXYGEN FRACTION UNDER PRESSURE TO PRODUCE THE LIDUID OXYGEN FRACTION WHICH IS VAPORIZED BY HEAT EXCHANGE WITH THE COMPRESSED MINOR AIR STREAM. 